Method and apparatus for controlling multi-fluid flow in a micro channel

ABSTRACT

A method and apparatus for controlling multi-fluid flow in a micro channel is disclosed. The apparatus has a first inlet for a first fluid; a second inlet for a second fluid; a first outlet; and a second outlet. The micro channel is operatively and fluidically connected to the first inlet, the second inlet, the first outlet and the second outlet. The micro channel is for receiving the first fluid and the second fluid under pressure-driven flow; there being an interface between the first fluid and the second fluid when in the micro channel. The apparatus also includes a pair of electrodes for having a first electric field applied thereto for a controlling the fluid flow velocity of the first fluid along the micro channel.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from U.S. Provisional PatentApplication No. 60/618,603 filed Oct. 15, 2004, which is herebyincorporated herein in its entirety for all purposes.

FIELD OF THE INVENTION

This invention relates to a method and apparatus for controllingmulti-fluid flow in a micro channel and refers particularly, though notexclusively, to such a method and apparatus that operates onelectrokinetic and hydrodynamic principles. In a preferred aspect thepresent invention relates to a method and apparatus for controlling aposition of an interface of fluids in the micro channel for switching,mixing and/or cytometering. In a more preferred aspect the presentinvention is also for controlling the form and position of theinterface.

BACKGROUND OF THE INVENTION

Most solid surfaces acquire an electrostatic charge when in contact withpolar liquids. As a result, a difference in potential is developedacross the interface between the negative and positive phases. Thecharged interface attracts ions of opposition charge (counter-ions) andrepels ions of like charge (co-ions) in the liquid. The arrangement ofcharges that occurs near the interface leads to the development of anelectric double layer. When a tangential electric field is applied alongthe capillary along which the liquid flows, liquids are pumped due toelectroosmostic flow. The two widely used methods for the transportationof a single fluid in microfluidics are electroosmostic flow, andpressure-driven flow.

In microfluidics, the Reynolds number is small and fluid flow islaminar. Laminar fluid diffusion interfaces are created when two or morestreams flow in parallel within a single micro-structure. Since theflows are laminar, there is no mixing between them. No mixing may bevery useful because only diffusion occurs between the different streamsof flow. Therefore, it is able to be used for extraction or separationin biological analysis. Diffusion-based microfluidic devices, such asthe T-sensor® and the H-filter® have been developed for commercial useby Micronics, Inc. of Redmond, Wash., USA.

The variable viscosity of biological fluids can be problematic when thetwo streams of flows have different viscosities. The fluid with higherviscosity will occupy a wider portion of the channel while having asmaller velocity; whereas the fluid with lower viscosity flows at alarger velocity within a narrow portion of the channel. The two fluidswill still have the same volumetric flow rate. The unmatched viscosityaffects diffusion due to differences in residence time. The averageresidence time of the more viscous fluid will increase, while that ofthe less viscous fluid will decrease. To overcome this problem, it hasbeen proposed to measure the viscosity of the fluid, and to add aviscosity-enhancing solute to the less viscous fluid. Another proposalis to control the ratio of the volumetric flow rate of the two fluids.By increasing the flow rate of the less viscous fluid, it is possible tomaintain the interface of the two streams at the center of the channel.However, the unmatched average residence time remains unsolved becausethe less viscous fluid flows even faster, and has even shorter averageresidence time within the channel.

SUMMARY OF THE INVENTION

In accordance with a first preferred aspect there is provided apparatusfor controlling fluid flow in a micro channel, the apparatus comprising:

(a) a first inlet for a first fluid;

(b) a second inlet for a second fluid;

(c) a first outlet;

(d) a second outlet;

(e) the micro channel being operatively and fluidically connected to thefirst inlet, the second inlet, the first outlet and the second outlet;the micro channel being for receiving the first fluid and the secondfluid under pressure-driven flow; there being a first interface betweenthe first fluid and the second fluid when in the micro channel; and

(f) a pair of electrodes for having a first electric field appliedthereto for a controlling the first fluid flow velocity along the microchannel.

According to a second aspect there is provided a method for controllingfluid flow in a micro channel, the method comprising:

(a) supplying a first fluid through a first inlet under pressure-drivenflow;

(b) supplying a second fluid through a second inlet underpressure-driven flow;

(c) the first fluid being able to flow along a micro channel to a firstoutlet;

(d) the second fluid being able to flow along the micro channel to asecond outlet;

(e) the micro channel being operatively and fluidically connected to thefirst inlet, the second inlet, the first outlet and the second outlet;there being a first interface between the first fluid and the secondfluid when in the micro channel; and

(f) applying an electric field to a pair of electrodes for controllingthe first fluid flow velocity along the micro channel.

The micro channel has a width, the first electric field may also beingfor controlling the location of the first interface across the width,and residence time of the first and second fluids in the micro channel.

The first pair of electrodes may comprise a first electrode and a secondelectrode, the first electrode being in the first inlet and the secondelectrode being in the first outlet.

There may also be a third inlet also for a third fluid, the second inletbeing between the first inlet and the third inlet, there being a secondinterface between the second fluid and the third fluid; and a thirdoutlet; the third inlet and the third outlet being operatively andfluidically connected to the micro channel.

A second pair of electrodes for having a second electric field appliedthereto may be provided for controlling the third fluid flow velocityalong the micro channel from the third inlet. The second electric fieldmay also be for controlling the location of the second interface acrossthe width, and residence time of the first, second and third fluids inthe micro channel. The second pair of electrodes may comprise a firstelectrode and a second electrode, the first electrode being in the thirdinlet and the second electrode being in the third outlet.

There may also be a fourth outlet operatively and fluidically connectedto the micro channel. The second electrode of the second pair ofelectrodes may be in the fourth outlet.

There may also be a fifth outlet operatively and fluidically connectedto the micro channel. The second electrode of the second pair ofelectrodes may be in the fifth outlet.

The first electric field and the second electric field may be able to becontrolled for directing the second fluid to at least one of: the firstoutlet, the second outlet, the third outlet, the fourth outlet and thefifth outlet.

The method and apparatus may be used for at least one selected from thegroup consisting of: an electrokinetic flow switch, a micromixer, amicro-flow cytometer, an interface position controller, and anin-channel fluidic lens.

At least one fourth inlet may be provided that is operatively andfluidically connected to the micro channel and being for a fourth fluid.There may be one fourth inlet between the second and third inlets.Alternatively, there may be a pair of fourth inlets; a first of the pairof fourth inlets may be located between the first and second inlets, anda second of the pair of fourth inlets may be located between the secondinlet and the third inlet. The fourth fluid may be a protection fluidfor separating the first fluid from the second and third fluids.Alternatively, the fourth fluid may be two sample fluids, the secondfluid being a protection fluid for separating the two sample fluids.

The first and second electric fields may be controlled for narrowing astream width of the second fluid for flow focusing of the second fluid.The method and apparatus may be for mixing at the micro scale, the firstand second electric fields may be used for narrowing the stream width ofthe second and fourth fluids for controlling diffusion path anddiffusion time.

A controller may be provided for controlling at least one of the firstelectric field and the second electric field for controlling thelocation of at least one of the location of the first interface and thelocation of the second interface.

There may be provided a pair of additional electrodes axially of themicro channel, and a pair of further electrodes at the top and bottom ofthe micro channel, the further electrodes being for controlling a curvedshape of the first interface, and the additional electrodes being forcontrolling the focal length and position of the curved shape.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the present invention may be fully understood and readilyput into practical effect, there shall now be described by way ofnon-limitative example only preferred embodiments of the presentinvention, the description being with reference to the accompanyingillustrative drawings.

In the drawings:

FIG. 1 is a schematic top view of a preferred microchannel arrangement;

FIG. 2 is an enlarged vertical cross-sectional view along the lines andin the direction of arrows 2-2 on FIG. 1;

FIG. 3 is two graphs of the relationship between NaCl holdup and (a)different applied voltage for the same volumetric flow rates, and (b)volumetric flow rate under the same applied voltage;

FIG. 4 is a schematic illustration of a second preferred from of flowswitch at a first operational state;

FIG. 5 is a schematic illustration of the second preferred form of flowswitch at a second operational state;

FIG. 6 is a schematic illustration of the second preferred form of flowswitch at a third operational state;

FIG. 7 is a schematic illustration of the second preferred form of flowswitch at a fourth operational state;

FIG. 8 is a schematic illustration of the second preferred form of flowswitch at a fifth operational state;

FIG. 9 is a schematic illustration of the third preferred form of flowswitch at a first operational state;

FIG. 10 is a schematic illustration of the third preferred form of flowswitch at a second operational state;

FIG. 11 is a schematic illustration of a fourth preferred form of flowswitch at a first operational state;

FIG. 12 is a schematic illustration of a fourth preferred form of flowswitch at a second operational state;

FIG. 13 is a schematic illustration of a fourth preferred form of flowswitch at a third operational state;

FIG. 14 is a schematic illustration of a micro mixer;

FIG. 15 is a schematic illustration of a microflow cytometer;

FIG. 16 is a schematic illustration of an interface position controller;and

FIG. 17 is a schematic illustration of an in-channel fluidic lens.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The first embodiment is shown in FIGS. 1 and 2 and includes an H-shapedmicro fluidics structure 10, syringes 31, 32 driven by pumps (notshown), and electrodes 14 for the application of electric field.Preferably the electrodes 14 are of a metal such as, for exampleplatinum. Each syringe 31, 32 supplies a single fluid of the two fluids16, 17 with the two fluids 16, 17 flowing side-by-side in a straightmicrochannel 20 from left to right. The flow is under the action ofpressure from the syringes 31, 32. A and C are the inlets for the twofluids 16, 17 and B and D are the outlets for the collection of theproducts, or the wastes. Between A and B, electrodes 14 are inserted forthe application of the electric field and are supplied by power supply22. The electric filed from A to B is positive, and from B to A isnegative. The straight portion 20 of the H-shaped structure 10 may haveany suitable size and configuration such as, for example, a crosssectional area of 1000 μm×100 μm, and a length of 5 mm. This gives awidth to depth rate of 10:1.

Solution 16 may be, for example, an aqueous NaCl solution (concentration0.7×10⁻³M) and solution 17 may be, for example, an aqueous glycerol(volume concentration 14%). The solutions 16, 17 are introduced throughinlets A and C respectively. The schematic cross-sectional view of thetwo fluids flow inside the straight channel is shown in FIG. 2. Thewidths occupied by the NaCl 16 and aqueous glycerol 17 are denoted as w₂and w₁ respectively. The holdup of the NaCl 16, e₂, is the ratio of thearea occupied by the NaCl 16 to the whole area of the cross-section ofthe channel 20. As the height is common, this becomes:$e_{2} = {\frac{w_{2}}{w_{1} + w_{2}}.}$

Similarly, the holdup of the aqueous glycerol 17 is e₁=1−e₂.

When the two fluids 16, 17 are in contact with the wall of the channel20, the negatively charged surface 20 will influence the distribution offree ions in the NaCl solution 16 to form an electrical double layernear the channel wall 20. But the aqueous glycerol 17 will onlyminimally form an electrical double layer as there are few free ions.Thus the electroosmotic flow will only affect the NaCl solution 16. Whena positive voltage is applied between A and B (A as the positiveelectrode, B as the negative electrode), the electroosmotic force willforce the NaCl solution 16 to flow in the same direction as thepressure-driven flow. If the negative electric field is applied (A isnegative, B is positive), an opposite electroosmotic flow will resultwhich is against the pressure-driven flow.

A fluorescent dye such as, for example, fluorescein disodium saltC₂₀H₁₀Na₂O₅, (also called Acid Yellow 73) may be added to the NaClsolution 16 for image collection. When the fluorescein is illuminated bya mercury lamp, a coupled charge device (CCD) camera or other similardevice may be used for image capturing to enable measurements to betaken. The same volumetric flow rates of the two inlet flows A and C maybe ensured through the use of identical syringes driven by a singlesyringe pump.

The parameters considered in the graphs of FIG. 3 are inlet volumetricflow rates, and electric voltage applied between A and B. The holdup ofthe NaCl solution 16 was obtained by normalizing its width w₂ to thewhole channel width (w₂₊w₁). As shown in FIG. 3(a), when the electricfield changes in magnitude and direction, the holdup of NaCl solution 16changes accordingly. When no voltage is applied across A and B, the flowis simply a pressure-driven two-phase flow. As the aqueous glycerol 17is about 1.5 times more viscous than the NaCl solution 16, the lessviscous NaCl solution 16 occupies a smaller portion of the channel 20width. The NaCl solution has a holdup of 0.35 without an externallyapplied voltage, as shown in FIG. 3(a). When a negative electric fieldis applied across A and B, the holdup of the NaCl solution 16 increasesas the electroosmotic flow is against the pressure-driven flow by theuse of a negative electric field. One explanation for this is that theNaCl solution 16 is becoming more “viscous” due to the electroosmoticeffect. As such it occupies a larger proportion of the width of channel20−w₂ increases and w₁ decreases. The holdup of the NaCl solution 16increases with an increase in the negative electric field.

Due to the same pressure drop across E and F, in order to achieve thesame volumetric flow rates, the more viscous fluid has to spread to alarger width, i.e. a higher liquid holdup. When a positive electricfield is applied, the NaCl solution 16 has a lower “viscosity”, sincethe electroosmotic flow is the same direction as the pressure-drivenflow so that the electroosmotic effect aids the flow of the NaClsolution 16.

FIG. 3(a) also shows that as the inlet volumetric flow rates of the twofluids increase, the electroosmotic flow effect on the pressure-drivenflow weaken. At the flow rate of 1.2 ml/h, the holdup of NaCl, e₂,remains constant even though the voltage varies from −0.8 kV to 0.6 kV.For typical electroosmotic flows, in which hundreds of volts percentimeter of electric field are applied, the resultant flow rate is ofthe order 0.1 to a few mm/s. But for pressure-driven flow inmicrochannels, the flow rate can be controlled over a wider range. Whenthe pressure-driver flow rate is set at 0.4 ml/h, the average velocityfor the NaCl solution 16 through the channel 20 is 3.17 mm/s with a noexternal applied electric field. This is comparable to that fromelectroosmotic flow. FIG. 3(a) shows that by adjusting the electricfield the position of interface 24 between the two fluids can becontrolled. As such, variation of the NaCl solution 16 holdup e₂, from0.25 to 0.50 is able to be controlled.

The relationship between the NaCl holdup, e₂ at different flow ratesunder the fixed electric field is shown in FIG. 3(b). Holdup e₂ remainsthe same (0.35) for different volumetric flow rates in the absence of anexternally applied electric field. This is because the volumetric flowrates ratio between the two fluids is kept unchanged at 1:1. As the flowrate increases, holdup e₂ converges to a constant value, 0.35. This isthe value without an externally applied electric field. The reason forthis is that the larger, pressure-driven flow speed makes theelectroosmotic effect virtually negligible.

Therefore, by adjusting the magnitude and the direction of the appliedelectric field the position of interface 24 between the two fluids 16,17 can be controlled, as can be the average residence time for thefluids. The H-shaped microfluidics structure 10 can therefore be used asa diffusion-based analysis device as it provides the same averageresidence time for the two fluids.

A second preferred from of microfluidic flow switch is shown in a FIG.4. The microfluidics device 400 has three inlets 401, 402 and 403 withrespective syringes 431, 432 and 433; and five outlets, 411 to 415.Inlets 401 and 403 are spaced apart and are for the introduction ofcontrol fluids 416 and 418 such as, for example, aqueous NaCl. Thesample fluid 417, which can be a biological fluid of interest, isintroduced from inlet 402 between the other two inlets 401, 403. A firstpair of electrodes 421 is located between inlet 401 and outlet 411, anda second pair of electrodes 422, are located between inlet 403 andoutlet 415 for the application of electric fields. The first electrodes421 are supplied by a first power supply 423; and the second electrodes422 are supplied by a second power supply 424.

Without changing the flow rate, the spread widths of the three laminarstreams of fluids 416, 417 and 418 can be adjusted by adjusting thedirection and strength of the electric-field, based on the workingprinciple described above. The sample fluid 417 can therefore be guidedinto different outlets by controlling the direction and strength of thevoltage applied to electrodes 421 and 422.

With FIG. 4, the electrodes 421 and 422 have electric fields that areapplied to them equally so the fluids 416 and 418 will occupy an equalportion of the width of channel 420. In that way the sample fluid 417 isguided down the centre of the channel 420 and thus exits through thecentrally-aligned outlet 413.

As shown in FIG. 5, if the electrodes 421 have a positive electric fieldapplied to them and electrodes 422 have a negative field applied tothem, the fluid 416 will occupy a reduced portion of the width ofchannel 420, and fluid 418 will occupy an increased portion of the widthof channel 420, thereby guiding the sample fluid to outlet 412. Asimilar effect may be achieved by having a strong, positive electricfield applied to electrodes 421 and no electric field applied toterminals 422. The effect is created by having the field applied toelectrodes more positive than that applied to electrodes 422.

FIG. 6 is the reverse of FIG. 5, so that sample fluid 417 flows tooutlet 414, and FIG. 7 is the same as FIG. 5 except that the differencein the applied electric fields is greater so that sample fluid flows tooutlet 411.

To get a sample fluid of a high purity, the electric fields can beadjusted in such a way that the sample fluid 417 width is slightlylarger then the outlet width.

Besides flow switching, the device can be used for the purposes of flowfocusing. It is possible to squeeze the sample fluid 417 into a verythin flow to allow only a single cell or several cells to pass as isshown in FIG. 8. This is useful for cell detection. If the electricfield is remotely controlled such as, for examples, by using a computer,it may be possible to achieve a programmable sample injection device orprogrammable dispensing device. The device can also be used as a valve,since the desired outlet can be selected by controlling the electricfield.

To reduce diffusion or reaction between the control fluid 416, 418 andthe sample fluid 417, another protection fluid 419 can be introduced toseparate the two, as shown in FIG. 9. Preferably, the protection fluid419 is relatively inert with both the control fluid 416, 418 and thesample fluid 417. The protection fluid 419 can be introduced by extrasyringes 434, 435 and respective inlets 404, 405.

Also, it is possible to switch more than one sample fluid 417 as shownin FIG. 10. Between the two sample fluids 417(a) and 417(b), a bufferfluid or a protection fluid 419 is introduced for separation of the twosample fluids 417(a) and 417(b).

Other design based on this working principle is possible. FIGS. 11 to 13show a Y-shaped flow switch under different work modes, e.g. switchingsample fluid to one or more outlets. In FIG. 11, the Y-shapedmicrofluidic flow switch has two inlets 401, 402 and four outlets 411 to414. The control fluid 416 and the sample fluid 417 are introduced frominlets 401 and 402. The electric field is applied through two electrodes421 inserted between inlet 401 and outlet 411. The sample fluid 417 canbe directed to the outlets 412, 413 and 414. The flow switch directs thesample fluid to outlet 412 as shown. In FIG. 12, the sample fluid isbeing passed to outlets 412, 413 and FIG. 13 it is passed to all outlets411 to 414. This may be simultaneously, or sequentially.

FIG. 14 shows the use of the device as a micromixer. The diffusiondistance, according to the square dependency, affects the diffusion timebetween the laminar flows of two sample fluids 417(a) and 417(b). Asdiffusion is the main mechanism through which mixing occurs between thetwo laminar streams, by adjusting the electric field across the controlfluids 416 and 418, it is possible for the two sample fluids to besqueezed into a narrow stream to thus reduce the diffusion path anddiffusion time and increase the mixing efficiency.

FIG. 15 shows its use as a micro flow cytometer. A conventionalmicro-flow cytometer uses hydrodynamic focusing. Instead of focusing thesample flow hydrodynamically through the sheath flow rate, by combiningthe pressure driven and the electrokinetic effects, a micro-flowcytometer capable of focusing the cells in the sample fluid 417 iscreated. The fluid flow along channel 420 will be smaller in width thanthe inlet 403, and is preferably the same as, or only slightly greaterthan outlet 413. In this way the focusing takes place along channel 420.

Although the electrodes 14, 421 and 422 are described and illustrated asbeing in the inlets and outlets, they may be located in channel 20, 420adjacent the inlets and outlets; or at the junction of the inlets andthe channel, and/or the junction of the outlets and the channel.

FIG. 16 illustrates a controller for determining and controlling thepositions of the interfaces. When the fluids in the channel 1620 areexcited with a laser 1640, fluorescent light signals are emitted. Aband-gap filter 1642 is placed on the other side of the channel 1620 sothat only light of the emitted wavelength is passed to a CCD array 1644,or other photosensor. The fluorescent signal will detect the presence ofthe fluid interfaces and thus enable the position of the fluidinterfaces to be determined as the output signal 1646 is proportional tothe bright area of the channel 1620. The interface position is comparedto the desired position 1648 in a controller 1650, and, if they aredifferent, the controller 1650 outputs a control signal 1652 that isreceived by an amplifier 1622. The power supply to the terminals 1614 isadjusted to adjust the applied electric field to channel 1620 therebycontrolling the interface position.

FIG. 17 illustrates an in-channel fluidic lens. Here two additionalelectrodes 1760 and 1762 are used for axial control; and two furtherelectrodes 1764 and 1766 are placed at the top and bottom of the channel1720 at the detection area of the channel 1720. The electrodes may bemade of a transparent material such as, for example, Indium Tin Oxide.The two further electrodes 1764 and 1766 are controlled by an appliedpotential that, in turn, controls the contact angle 1768. Therefore, theinterface 1770 becomes curved, as shown. The curved interface 1770 actsas a cylindrical lens, and serves to focus the incoming excitation laser1772 to a sheet with high intensity. This allows for a largefluorescence detection area within channel 1720, and for the emittedsignal 1776 to have a higher intensity. The focal length and position1774 can be controlled by the potential applied to the additionalterminals 1760, 1762. Therefore, by selective excitation of the fourterminals, 1760, 1762, 1764 and 1766 improved performance may result.

Whilst there has been described in the foregoing description preferredembodiments of the present invention, it will be understood by thoseskilled in the technology concerned that many variations ormodifications in details of design or construction may be made withoutdeparting from the present invention.

1. Apparatus for controlling fluid flow in a micro channel, theapparatus comprising: (a) a first inlet for a first fluid; (b) a secondinlet for a second fluid; (c) a first outlet; (d) a second outlet; (e)the micro channel being operatively and fluidically connected to thefirst inlet, the second inlet, the first outlet and the second outlet;the micro channel being for receiving the first fluid and the secondfluid under pressure-driven flow; there being a first interface betweenthe first fluid and the second fluid when in the micro channel; and (f)a pair of electrodes for having a first electric field applied theretofor a controlling the first fluid flow velocity along the micro channel.2. Apparatus as claimed in claim 1, wherein the micro channel has awidth, the first electric field also being for controlling the locationof the first interface across the width, and residence time of the firstand second fluids in the micro channel.
 3. Apparatus as claimed in claim1, wherein the first pair of electrodes comprises a first electrode anda second electrode, the first electrode being in the first inlet and thesecond electrode being in the first outlet.
 4. Apparatus as claimed inclaim 2, further comprising: (g) a third inlet for a third fluid, thesecond inlet being between the first inlet and the third inlet, therebeing a second interface between the third fluid and the second fluid;and (h) a third outlet, the third inlet and the third outlet beingoperatively and fluidically connected to the micro channel.
 5. Apparatusas claimed in claim 4, further comprising: (i) a second pair ofelectrodes for having second electric field applied thereto forcontrolling the third fluid flow velocity along the micro channel fromthe third inlet.
 6. Apparatus as claimed in claim 5, wherein the microchannel has a width, the second electric field also being forcontrolling the location of the second interface across the width, andresidence time of the first, second and third fluids in the microchannel.
 7. Apparatus as claimed in claim 5, wherein the second pair ofelectrodes comprises a first electrode and a second electrode, the firstelectrode being in the third inlet and the second electrode being in thethird outlet.
 8. Apparatus as claimed in claim 4, further comprising:(j) a fourth outlet operatively and fluidically connected to the microchannel.
 9. Apparatus as claimed in claim 8, further comprising: (k) asecond pair of electrodes for having second electric field appliedthereto for controlling the third fluid flow velocity along the microchannel from the third inlet.
 10. Apparatus as claimed in claim 9,wherein the micro channel has a width, the second electric field alsobeing for controlling the location of the second interface across thewidth, and residence time of the first, second and third fluids in themicro channel.
 11. Apparatus as claimed in claim 9, wherein the secondpair of electrodes comprises a first electrode and a second electrode,the first electrode being in the third inlet and the second electrodebeing in the fourth outlet.
 12. Apparatus as claimed in claim 8, furthercomprising: (l) a fifth outlet operatively and fluidically connected tothe micro channel.
 13. Apparatus as claimed in claim 12, furthercomprising: (m) a second pair of electrodes for having second electricfield applied thereto for controlling the third fluid flow velocityalong the micro channel from the third inlet.
 14. Apparatus as claimedin claim 13, wherein the micro channel has a width, the second electricfield also being for controlling the location of the second interfaceacross the width, and residence time of the first, second and thirdfluids in the micro channel.
 15. Apparatus as claimed in claim 13,wherein the second pair of electrodes comprises a first electrode and asecond electrode, the first electrode being in the third inlet and thesecond electrode being in the fifth outlet.
 16. Apparatus as claimed inclaim 5, wherein the first electric field and the second electric fieldare able to be controlled for directing the second fluid to at least oneof: the first outlet, the second outlet and the third outlet. 17.Apparatus as claimed in claim 9, wherein the first electric field andthe second electric field are able to be controlled for directing thesecond fluid to at least one of: the first outlet, the second outlet,the third outlet and the fourth outlet.
 18. Apparatus as claimed inclaim 13, wherein the first electric field and the second electric fieldare able to be controlled for directing the second fluid to at least oneof: the first outlet, the second outlet, the third outlet, the fourthoutlet and the fifth outlet.
 19. Apparatus as claimed in claim 1,wherein the apparatus is used for at least one selected from the groupconsisting of: an electrokinetic flow switch, a micromixer, a micro-flowcytometer, an interface position controller, and an in-channel fluidiclens.
 20. Apparatus as claimed in claim 4, further comprising at leastone fourth inlet operatively and fluidically connected to the microchannel and being for a fourth fluid.
 21. Apparatus as claimed in claim20, wherein there is one fourth inlet between the second inlet and thethird inlet.
 22. Apparatus as claimed in claim 20, wherein there is apair of fourth inlets, a first of the pair of fourth inlets beinglocated between the first and second inlets, and a second of the pair offourth inlets being located between the second inlet and the thirdinlet.
 23. Apparatus as claimed in claim 22, wherein the fourth fluid isa protection fluid for separating the first fluid from the second andthird fluids.
 24. Apparatus as claimed in claim 22, wherein the fourthfluid comprises at least one sample fluid, the second fluid being aprotection fluid for separating the at least one sample fluid. 25.Apparatus as claimed in claim 5, wherein the first and second electricfields are able to be controlled for narrowing a stream width of thesecond fluid for flow focusing of the second fluid.
 26. Apparatus asclaimed in claim 21, wherein apparatus is used as a micro-mixer, thefirst and second electric fields being able to be controlled fornarrowing the stream width of the second and fourth fluids forcontrolling diffusion path and diffusion time.
 27. Apparatus as claimedin claim 5, further comprising a controller for controlling at least oneof the first electric field and the second electric field forcontrolling the location of at least one of the location of the firstinterface and the location of the second interface.
 28. Apparatus asclaimed in claim 1, wherein there is provided a pair of additionalelectrodes axially of the micro channel, and a pair of furtherelectrodes at the top and bottom of the micro channel, the furtherelectrodes being for controlling a curved shape of the first interface,and the additional electrodes being for controlling the focal length andposition of the curved shape.
 29. A method for controlling fluid flow ina micro channel, the method comprising: (a) supplying a first fluidthrough a first inlet under pressure-driven flow; (b) supplying a secondfluid through a second inlet under pressure-driven flow; (c) the firstfluid being able to flow along a micro channel to a first outlet; (d)the second fluid being able to flow along the micro channel to a secondoutlet; (e) the micro channel being operatively and fluidicallyconnected to the first inlet, the second inlet, the first outlet and thesecond outlet; there being a first interface between the first fluid andthe second fluid when in the micro channel; and (f) applying an electricfield to a pair of electrodes for controlling the first fluid flowvelocity along the micro channel.
 30. A method as claimed in claim 29,wherein the micro channel has a width, the first electric field alsobeing used for controlling the location of the first interface acrossthe width, and residence time of the first and second fluids in themicro channel.
 31. A method as claimed in claim 30, wherein the firstpair of electrodes comprises a first electrode and a second electrode,the first electrode being in the first inlet and the second electrodebeing in the first outlet.
 32. A method as claimed in claim 29, furthercomprising: (g) also supplying the third fluid through a third inlet,the second inlet being between the first inlet and the third inlet,there being a second interface between the third fluid and the secondfluid; and (h) the third fluid supplied through the third inlet beingable to flow along the micro channel to a third outlet; the third inletand the third outlet being operatively and fluidically connected to themicro channel.
 33. A method as claimed in claim 32, further comprising:(i) applying a second electric field to a second pair of electrodes forcontrolling the third fluid flow velocity along the micro channel fromthe third inlet.
 34. A method as claimed in claim 33, wherein the microchannel has a width, the second electric field also being forcontrolling the location of the second interface across the width, andresidence time of the first, second and third fluids in the microchannel.
 35. A method as claimed in claim 34, wherein the second pair ofelectrodes comprises a first electrode and a second electrode, the firstelectrode being in the third inlet and the second electrode being in thethird outlet.
 36. A method as claimed in claim 32, further comprising:(j) a fourth outlet operatively and fluidically connected to the microchannel; (k) applying a second electric field to a second pair ofelectrodes for controlling the third fluid flow velocity along the microchannel from the third inlet.
 37. A method as claimed in claim 36,wherein the micro channel has a width, the second electric field alsobeing for controlling the location of the second interface across thewidth, and residence time of the first, second and third fluids in themicro channel.
 38. A method as claimed in claim 36, wherein the secondpair of electrodes comprises a first electrode and a second electrode,the first electrode being in the third inlet and the second electrodebeing in the fourth outlet.
 39. A method as claimed in claim 36, furthercomprising: (l) a fifth outlet operatively and fluidically connected tothe micro channel; (m) applying a second electric field to a second pairof electrodes for controlling the third fluid flow velocity along themicro channel from the third inlet.
 40. A method as claimed in claim 39,wherein the micro channel has a width, the second electric field alsobeing for controlling the location of the second interface across thewidth, and residence time of the first, second and third fluids in themicro channel.
 41. A method as claimed in claim 39, wherein the secondpair of electrodes comprises a first electrode and a second electrode,the first electrode being in the third inlet and the second electrodebeing in the fifth outlet.
 42. A method as claimed in claim 33, whereinthe first electric field and the second electric field are controlledfor directing the second fluid to at least one of: the first outlet, thesecond outlet and the third outlet.
 43. A method as claimed in claim 36,wherein the first electric field and the second electric field arecontrolled for directing the second fluid to at least one of: the firstoutlet, the second outlet, the third outlet and the fourth outlet.
 44. Amethod as claimed in claim 39, wherein the first electric field and thesecond electric field are controlled for directing the second fluid toat least one of: the first outlet, the second outlet, the third outlet,the fourth outlet and the fifth outlet.
 45. A method as claimed in claim29, wherein the method is used for at least one selected from the groupconsisting of: an electrokinetic flow switch, a micromixer, a micro-flowcytometer, an interface position controller, and an in-channel fluidiclens.
 46. A method as claimed in claim 32, further comprising supplyinga fourth fluid through at least one fourth inlet operatively andfluidically connected to the micro channel.
 47. A method as claimed inclaim 46, wherein there is one fourth inlet between the second inlet andthe third inlet.
 48. A method as claimed in claim 46, wherein there is apair of fourth inlets, a first of the pair of fourth inlets beinglocated between the first and second inlets, and a second of the pair offourth inlets being located between the second inlet and the thirdinlet.
 49. A method as claimed in claim 48, wherein the fourth fluid isa protection fluid for separating the first fluid from the second andthird fluids.
 50. A method as claimed in claim 48, wherein the fourthfluid comprises two sample fluids, the second fluid being a protectionfluid for separating the two sample fluids.
 51. A method as claimed inclaim 34, further comprising controlling the first and second electricfields for narrowing a stream width of the second fluid for flowfocusing of the second fluid.
 52. A method as claimed in claim 47,wherein the method is for mixing at a micro scale, the method furthercomprising controlling the first and second electric fields fornarrowing the stream width of second and fourth fluids for controllingdiffusion path and diffusion time.
 53. A method as claimed in claim 33,further comprising controlling at least one of the first electric fieldand the second electric field for controlling at least one of: the firstelectric field and the second electric field, for controlling thelocation of at least one of: the location of the first interface and thelocation of the second interface.
 54. A method as claimed in claim 29,wherein there is provided a pair of additional electrodes axially of themicro channel, and a pair of further electrodes at the top and bottom ofthe micro channel, the further electrodes being for controlling a curvedshape of the first interface, and the additional electrodes being forcontrolling the focal length and position of the curved shape.